问题描述
我对列表副本有一些疑问:
I have some problem with a List copy:
因此,当我从'get_edge'
获得E0
后,我通过调用'E0_copy = list(E0)'
制作了E0
的副本.在这里,我想E0_copy
是E0
的深层副本,并且我将E0_copy
传递给'karger(E)'
.但是在主要功能上.
为什么'print E0[1:10]'
在for循环之前的结果与for循环之后的结果不同?
So After I got E0
from 'get_edge'
, I make a copy of E0
by calling 'E0_copy = list(E0)'
. Here I guess E0_copy
is a deep copy of E0
, and I pass E0_copy
into 'karger(E)'
. But in the main function.
Why does the result of 'print E0[1:10]'
before the for loop is not the same with that after the for loop?
下面是我的代码:
def get_graph():
f=open('kargerMinCut.txt')
G={}
for line in f:
ints = [int(x) for x in line.split()]
G[ints[0]]=ints[1:len(ints)]
return G
def get_edge(G):
E=[]
for i in range(1,201):
for v in G[i]:
if v>i:
E.append([i,v])
print id(E)
return E
def karger(E):
import random
count=200
while 1:
if count == 2:
break
edge = random.randint(0,len(E)-1)
v0=E[edge][0]
v1=E[edge][1]
E.pop(edge)
if v0 != v1:
count -= 1
i=0
while 1:
if i == len(E):
break
if E[i][0] == v1:
E[i][0] = v0
if E[i][1] == v1:
E[i][1] = v0
if E[i][0] == E[i][1]:
E.pop(i)
i-=1
i+=1
mincut=len(E)
return mincut
if __name__=="__main__":
import copy
G = get_graph()
results=[]
E0 = get_edge(G)
print E0[1:10] ## this result is not equal to print2
for k in range(1,5):
E0_copy=list(E0) ## I guess here E0_coypy is a deep copy of E0
results.append(karger(E0_copy))
#print "the result is %d" %min(results)
print E0[1:10] ## this is print2
推荐答案
E0_copy
不是深层副本.您不能使用list()
进行深拷贝(list(...)
和testList[:]
都是浅拷贝).
E0_copy
is not a deep copy. You don't make a deep copy using list()
(Both list(...)
and testList[:]
are shallow copies).
您使用 copy.deepcopy(...)
进行深度复制列表
You use copy.deepcopy(...)
for deep copying a list.
deepcopy(x, memo=None, _nil=[])
Deep copy operation on arbitrary Python objects.
请参见以下代码段-
>>> a = [[1, 2, 3], [4, 5, 6]]
>>> b = list(a)
>>> a
[[1, 2, 3], [4, 5, 6]]
>>> b
[[1, 2, 3], [4, 5, 6]]
>>> a[0][1] = 10
>>> a
[[1, 10, 3], [4, 5, 6]]
>>> b # b changes too -> Not a deepcopy.
[[1, 10, 3], [4, 5, 6]]
现在请参阅deepcopy
操作
>>> import copy
>>> b = copy.deepcopy(a)
>>> a
[[1, 10, 3], [4, 5, 6]]
>>> b
[[1, 10, 3], [4, 5, 6]]
>>> a[0][1] = 9
>>> a
[[1, 9, 3], [4, 5, 6]]
>>> b # b doesn't change -> Deep Copy
[[1, 10, 3], [4, 5, 6]]
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